Preliminary results from the single stage pressure reduction electrospray ionization instrument (API-1) show that very large water clusters, containing more than 25 water molecules and a single proton can be observed. These clusters can be reliably observed, albeit not demononstrably from equilibrium conditions as yet. The reliability can, however, be used to calibrate the instrument's mass scale. Such an approach to claibration could have significant advantages to analytical chemists because it permits mass scale calibration without introduction contaminating molecules into the instruemnt. Similar large clusters containing 15-20 water molecules have been obtained with the tetraphenyl arsonium ion, although not yet from equilibrium conditions.